In general, vinyl chloride-based polymers contain at least 50% of vinyl chloride and are used in a variety of applications since they are inexpensive, easy to control hardness, and applicable to most processing apparatuses. Moreover, vinyl chloride-based polymers have excellent physical and chemical properties such as mechanical strength, weather resistance, and chemical resistance, and are thus being widely used in various fields such as building materials, household goods, automobile interior materials, and decorative materials. However, vinyl chloride-based polymers have various processing problems in that, for example, moldable temperature ranges are narrow due to processing temperatures close to the thermal decomposition temperatures thereof and it takes a long time to become a molten state.
To solve the above-mentioned problems, there are various known methods, such as adding a plasticizer to a vinyl chloride-based polymer, using a vinyl chloride-based copolymer of a vinyl chloride-based monomer and other monomers such as vinyl acetate, and using a blend of a vinyl chloride-based polymer and other resin components.
However, the above methods have problems in that it is difficult to improve the processability of vinyl chloride-based polymers while maintaining excellent physical properties and chemical properties inherent thereto. For example, adding a plasticizer to a vinyl chloride-based polymer or using a vinyl chloride-based copolymer of a vinyl chloride-based monomer and other monomers such as vinyl acetate significantly changes physical properties of molded articles.
In addition, the method in which other resin components are blended with a vinyl chloride-based polymer generally lowers the melt viscosity during molding process and thus lowers the processing temperature. Furthermore, kneading energy consumed by flows during the process results in insufficient gelation of a vinyl chloride-based polymer, which degrades physical properties compared to a sufficiently-gelled vinyl chloride-based polymer.
Meanwhile, in order to promote the gelation of a vinyl chloride-based polymer during molding process or improve the appearance of molded articles, a method in which a copolymer including methyl methacrylate-based as a main component is added to a vinyl chloride-based polymer has been proposed. This method has advantages in that the processability of a highly gelled vinyl chloride-based polymer can be improved while maintaining the mechanical properties and transparency of molded articles, and the generation of air marks can be prevented when a blend of the copolymer and a vinyl chloride-based polymer is molded into a sheet by calendering. However, this method has the problem of causing flow marks on the sheet surface and thus degrading the quality of molded articles.
Furthermore, appearance characteristics have been recently emphasized with the improvement of molding techniques or blending techniques, and there have been a growing demand for developing techniques capable of preventing the generation of fish-eyes and improving transparency.
For example, U.S. Pat. No. 5,204,421 discloses a preparation method in which a dispersant having a degree of hydration of 20-55% is added in installments in the initial stage and the middle stage of polymerization, and indicates that a vinyl chloride polymer having excellent plasticizer absorption and a small number of fish-eyes can be prepared through the preparation method. U.S. Pat. No. 7,001,960 discloses a preparation method in which a dispersant having a degree of hydration of 20-57% is continuously added from the initial stage of polymerization to the middle stage of polymerization, and indicates that a vinyl chloride polymer having a small number of fish-eyes can be prepared through the preparation method. Korean patent publication No. 2013-0001428 discloses a method for suppressing the formation of scale in a polymerization reactor and thus preventing the generation of fish-eyes by adding a metal deactivator during a polymerization reaction. However, the above conventional methods have a negligible effect on the reduction of fish-eyes and cannot significantly improve the transparency of produced molded articles.
Therefore, in order to easily apply a vinyl chloride-based polymer to various fields, it is necessary to develop a technique capable of improving the processability of the vinyl chloride-based polymer such that the generation of fish-eyes can be suppressed and the appearance characteristics of produced molded articles can thus be improved.